1. Field of the Invention
This invention relates to a method of manufacturing semiconductor devices and, more specifically, to such a method with minimization of "bird beak" or oxide encroachment into the active device region.
2. Description of the Prior Art
In the prior art methods of manufacture of MOS and bipolar semiconductor devices, some of the processing steps require that portions of the silicon substrate be prevented from being oxidized during oxidation of other portions of the substrate device. In order to accomplish this selective oxidation inhibition feature, silicon nitride has been utilized as a mask in the regions wherein oxidation inhibition is to occur, such as the device active or moat region, to prevent oxidation of the substrate therebelow. This has caused a problem wherein the silicon nitride layer on the silicon layer or substrate develops too much stress on the silicon layer. Accordingly, the prior art has placed a stress relieving buffer between the silicon nitride layer and the silicon substrate, usually in the form of an oxide and preferably a silicon oxide. The oxide layer must be of a certain minimum thickness to perform its function or this leads to defects and leakage in the devices formed at the edge of the isolation region. A further problem caused by use of the buffer region is that enhanced oxidation is provided in the buffer region and forms a channel which develops what is known as "bird beak" or oxide encroachment into the active device region of the substrate. This encroachment creates problems in that it limits the usable active device area because, for example, in MOS devices, a certain gate device thickness is expected and that thickness should be constant up to the edge of the isolation region. This cannot be accomplished when oxide encroachment is present.
Many techniques have been attempted to eliminate the above described encroachment problem. The obvious approach is to decrease the oxide thickness and this has been attempted for many years but without success because the defect problem has always appeared. Other proposals have attempted to grow a nitride layer on the silicon which is deposited in such a way that it matches the stoichiometry of the silicon structure. The nitride layers that are normally used are deposited in a low pressure CVD reactor and there is a great deal of stress where the nitride contacts the silicon. It is therefor clear that, by these direct deposition techniques, it has not been possible to obtain nitride layers of more than about 50 Angstroms thickness whereas less than 100 Angstroms of silicon nitride will not mask a thick oxide growth. It is therefore apparent that eventually the nitride is consumed and converted and the active device region starts to oxidize.